DateFormatter: add iso-8601 formatter that handles : symbol in timezone.

The one we have only handles ISO-8601 dates formatted as '2014-06-02T00:00:00.000+0200'
But ISO-8601 dates can also be like '2014-06-02T00:00:00.000+02:00'
Merge pull request #70 from iZettle/isodate-with-timezone-kolon

DateFormatter: add iso-8601 formatter that handles : symbol in timezone.